Electrical contact element for a fuel cell having an ultra-thin conductive layer coating

ABSTRACT

An electrically conductive fluid distribution element for use in a fuel cell includes a conductive metal substrate and a layer of conductive non-metallic porous media. The conductive non-metallic porous media has an electrically conductive material deposited along a surface in one or more metallized regions and having an average thickness less than about 40 nm. The metallized regions improve electrical conductance at contact regions between the metal substrate and the fluid distribution media.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/566,909 filed on Dec. 5, 2006, which was a continuation-in-part ofU.S. patent application Ser. No. 10/704,015 filed on Nov. 7, 2003. Theentire disclosures of the above applications are hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to fuel cells, and more particularly toelectrically conductive fluid distribution elements and the manufacturethereof, for such fuel cells.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for electric vehiclesand other applications. One known fuel cell is the PEM (i.e., ProtonExchange Membrane) fuel cell that includes a so-called MEA(“membrane-electrode-assembly”) comprising a thin, solid polymermembrane-electrolyte having an anode on one face and a cathode on theopposite face. The anode and cathode typically comprise finely dividedcarbon particles, very finely divided catalytic particles supported onthe internal and external surfaces of the carbon particles, and protonconductive material intermingled with the catalytic and carbonparticles. The MEA is sandwiched between gas diffusion media layers anda pair of electrically conductive contact elements which serve ascurrent collectors for the anode and cathode, which may containappropriate channels and openings therein for distributing the fuelcell's gaseous reactants (i.e. H₂ and O₂/air) over the surfaces of therespective anode and cathode.

Bipolar PEM fuel cells comprise a plurality of the MEAs stacked togetherin electrical series while being separated one from the next by animpermeable, electrically conductive contact element known as a bipolarplate or septum. The bipolar plate has two working surfaces, oneconfronting the anode of one cell and the other confronting the cathodeon the next adjacent cell in the stack, and electrically conductscurrent between the adjacent cells. Contact elements at the ends of thestack contact only the end cells and are referred to as end plates.

Electrical contact elements are often constructed from electricallyconductive metal materials. In an H₂ and O₂/air PEM fuel cellenvironment, the bipolar plates and other contact elements (e.g., endplates) are in constant contact with highly acidic solutions (pH 3-5)and operate in a highly oxidizing environment, being polarized to amaximum of about +1 V (vs. the normal hydrogen electrode). On thecathode side the contact elements are exposed to pressurized air, and onthe anode side exposed to super atmospheric hydrogen. Unfortunately,many metals are susceptible to corrosion in the hostile PEM fuel cellenvironment, and contact elements made therefrom either dissolve (e.g.,in the case of aluminum), or form highly electrically resistive,passivating oxide films on their surface (e.g., in the case of titaniumor stainless steel) that increases the internal resistance of the fuelcell and reduces its performance. Further, maintaining electricalconductivity through the gas diffusion media to the contact elements isof great importance in maintaining the flow of electrical current fromeach fuel cell. Thus, there is a need to provide electrically conductiveelements that maintain electrical conductivity, resist the fuel cellhostile environment, and improve overall operational efficiency of afuel cell.

SUMMARY OF THE INVENTION

The present invention provides an electrically conductive fluiddistribution element for use in a fuel cell which comprises a conductivemetal substrate and a layer of conductive non-metallic porous mediahaving a surface facing the metal substrate. One or of more metallizedregions are formed on the surface of the layer, each metallized regioncontaining an electrically conductive material and having a averagethickness equal to about the diameter of one atom of the material. Theconductive metal substrate is arranged in contact with the metallizedregions to provide an electrically conductive path between the layer andthe conductive metal substrate.

In alternate preferred embodiments of the present invention, an assemblyfor use in a fuel cell comprises an electrically conductive metalsubstrate having a major surface, a layer of electrically conductiveporous fluid distribution media having a first and a second surface,wherein the first surface is in electrical contact with the majorsurface and the second surface confronts a membrane electrode assembly,and one or more metallized regions on the first and the second surfacesof the layer, each metallized region containing an electricallyconductive material and having an average thickness equal to about thediameter of one atom of the material. An electrical contact resistanceacross the metal substrate through the metallized regions to the layeris less than a comparative contact resistance across a similar metalsubstrate and a similar layer of fluid distribution media absent themetallized regions.

Other alternate preferred embodiments comprise an electricallyconductive fluid distribution element for a fuel cell, the elementcomprising a layer of electrically conductive porous media comprisingcarbon and one or more ultra-thin metallized regions along a surface ofthe layer, where the one or more metallized regions comprise anelectrically conductive material and have an average thickness equal toabout the diameter of one atom of the material.

Other preferred embodiments of the present invention comprise a methodfor manufacturing an electrically conductive element for a fuel cell,comprising depositing an electrically conductive material on a surfaceof an electrically conductive porous media to form one or moremetallized regions having an average thickness equal to about thediameter of one atom of the material. The surface having the metallizedregions is positioned adjacent to a metallic electrically conductivesubstrate. The substrate is contacted with the surface having themetallized regions to form an electrically conductive path between thesubstrate and the porous media.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic, exploded illustration of a PEM fuel cell stack(only two cells shown);

FIG. 2 is an exploded view of an exemplary electrically conductive fluiddistribution element useful with PEM fuel cell stacks;

FIG. 3 is a partial cross-sectional view in the direction of 3-3 of FIG.2;

FIG. 4 is a not-to-scale side-sectional drawing taken in the directionof line 4-4 of FIG. 1 showing one preferred embodiment of the presentinvention where the metallized regions correspond to the entire surfaceof the layer of porous media;

FIG. 5 is a not-to-scale partial side-sectional detailed view of asingle layer of porous media adjacent to a membrane electrode assemblyaccording to alternate preferred embodiments of the present inventionwhere the metallized regions are discrete;

FIG. 6 is a an illustration of a physical vapor deposition apparatusused to metallize a surface of a porous fluid distribution media with anelectrically conductive metal;

FIG. 7 is a graph comparing a measurement of contact resistance achievedthrough a 316L stainless steel plate contacting a porous fluiddistribution media having metallized regions along a contact surfaceaccording to the present invention with a prior art porous fluiddistribution media; and

FIG. 8 is a graph of contact resistance values achieved by anelectrically conductive element of the present invention having aseparator element with a flow field formed therein and a layer of porousmedia having a surface with metallized regions, as compared with a priorart conductive element assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

FIG. 1 depicts a two cell, bipolar fuel cell stack 2 having a pair ofmembrane-electrode-assemblies (MEAs) 4 and 6 separated from each otherby an electrically conductive fluid distribution element 8, hereinafterbipolar plate 8. The MEAs 4 and 6 and bipolar plate 8, are stackedtogether between stainless steel clamping plates, or end plates 10 and12, and end contact elements 14 and 16. The end contact elements 14 and16, as well as both working faces of the bipolar plate 8, contain aplurality of grooves or channels 18, 20, 22, and 24, respectively, fordistributing fuel and oxidant gases (i.e. H₂ and O₂) to the MEAs 4 and6. Nonconductive gaskets 26, 28, 30, and 32 provide seals and electricalinsulation between the several components of the fuel cell stack. Gaspermeable conductive materials are typically carbon/graphite diffusionpapers 34, 36, 38, and 40 that press up against the electrode faces ofthe MEAs 4 and 6. The end contact elements 14 and 16 press up againstthe carbon/graphite papers 34 and 40 respectively, while the bipolarplate 8 presses up against the carbon/graphite paper 36 on the anodeface of MEA 4, and against carbon/graphite paper 38 on the cathode faceof MEA 6. Oxygen is supplied to the cathode side of the fuel cell stackfrom storage tank 46 via appropriate supply plumbing 42, while hydrogenis supplied to the anode side of the fuel cell from storage tank 48, viaappropriate supply plumbing 44. Alternatively, ambient air may besupplied using a compressor or blower to the cathode side as an oxygensource and hydrogen to the anode from a methanol or gasoline reformer,or the like. Exhaust plumbing (not shown) for both the H₂ and O₂ sidesof the MEAs 4 and 6 will also be provided. Additional plumbing 50, 52,and 54 is provided for supplying liquid coolant to the bipolar plate 8and end plates 14 and 16. Appropriate plumbing for exhausting coolantfrom the bipolar plate 8 and end plates 14 and 16 is also provided, butnot shown.

FIG. 2 is an exploded view of an exemplary bipolar plate 56 that may beused in accordance with a first embodiment of the present invention. Thebipolar plate 56 comprises a first exterior metal sheet 58, a secondexterior metal sheet 60, and an interior spacer metal sheet 62interjacent the first metal sheet 58 and the second metal sheet 60. Theexterior metal sheets 58 and 60 are made as thin as possible and may beformed by stamping, or any other conventional process for shaping sheetmetal. The external sheet 58 has a first working face 59 on the outsidethereof which confronts a membrane electrode assembly (not shown) and isformed so as to provide a flow field 57. The flow field 57 is defined bya plurality of lands 64 which define therebetween a plurality of grooves66 which constitutes the “flow field” through which the fuel cell'sreactant gases (i.e. H₂ or O₂) flow in a meandering path from one side68 of the bipolar plate to the other side 70 thereof. When the fuel cellis fully assembled, the lands 64 press against the porous material,carbon/graphite papers 36 or 38 which, in turn, press against the MEAs 4and 6. For simplicity, FIG. 2 depicts only two arrays of lands andgrooves. In reality, the lands and grooves will cover the entireexternal faces of the metal sheets 58 and 60 that engage thecarbon/graphite papers 36 and 38. The reactant gas is supplied togrooves 66 from a manifold 72 that lies along one side 68 of the fuelcell, and exits the grooves 66 via another manifold 74 that liesadjacent the opposite side 70 of the fuel cell.

As best shown in FIG. 3, the underside of the sheet 58 includes aplurality of ridges 76 which define therebetween a plurality of channels78 through which coolant passes during the operation of the fuel cell.As shown in FIG. 3, the coolant channel 78 underlies each land 64 whilea reactant gas groove 66 underlies each ridge 76. Alternatively, thesheet 58 could be flat and the flow field formed in a separate sheet ofmaterial. Metal sheet 60 is similar to sheet 58. The internal face 61 ofsheet 60 is shown in FIG. 2. In this regard, there is depicted aplurality of ridges 80, defining therebetween, a plurality of channels82 through which coolant flows from one side 69 of the bipolar plate tothe other 71. Like sheet 58 and as best shown in FIG. 3, the externalside of the sheet 60 has a working face 63. Sheet 60 is formed so as toprovide a flow field 65. The flow field 65 is defined by a plurality oflands 84 thereon defining a plurality of grooves 86 which constitute theflow field 65 through which the reactant gases pass.

An interior metal spacer sheet 62 is positioned interjacent the exteriorsheets 58 and 60 and includes a plurality of apertures 88 therein topermit coolant to flow between the channels 82 in sheet 60 and thechannels 78 in the sheet 58 thereby breaking laminar boundary layers andaffording turbulence which enhances heat exchange with the inside faces90 and 92 of the exterior sheets 58 and 60, respectively. Thus, channels78 and 82 form respective coolant flow fields at the interior volumedefined by sheets 58 and 60. Alternate embodiments (not shown) comprisetwo stamped plates joined together by a joining process to form interiorcoolant from fields.

In FIG. 4, a membrane-electrode-assembly 100 (MEA) comprises a membrane102 sandwiched between an anode 104 and a cathode 106 which are boundedby an electrically-conductive material known as “diffusion media” orporous fluid distribution media 107. The porous media 107 is interposedbetween two current collectors separator plate substrates 113,115 andthe MEA 100 and serves to (1) distribute gaseous reactant over theentire face of the MEA 100, between and under the lands 131 of thecurrent collector 113,115, and (2) collect current from the MEA 100. Afirst fluid distribution media layer 108 is adjacent to the anode 104and a second fluid distribution media layer 110 is adjacent to thecathode 106. A first separator plate surface or substrate (e.g bipolarplate) 112 is in contact with the first fluid distribution media layer108, and a second separator plate surface 114 contacts the second fluiddistribution media layer 110. According to the present invention, it ispreferred that the fluid distribution media 107 and the first and secondsubstrates 113,115 are constructed of electrically conductive materialsand electrical contact is established therebetween at one or moreelectrical contact regions 116 where an electrically conductive path isformed between a substrate sheet (113 or 115) and the correspondingporous media (108 or 110).

Preferred materials of construction for the separator plate substrates113,115 include conductive metals, such as stainless steel, aluminum,and titanium, for example. The most preferred materials of constructionfor the separator plate substrates 113,115 are higher grades ofstainless steel/alloys that exhibit high resistance to corrosion in thefuel cell, such as, for example, 316L, 317L, 256 SMO, Alloy 276, andAlloy 904L.

According to the present invention, the porous fluid distribution media107 comprises an electrically conductive non-metallic composition. Firstexternal surfaces 117 of the fluid distribution media 107 refers tothose surfaces of the first and second fluid distribution media layers108, 110 which contact the substrate sheets 113,115. Second externalsurfaces 118 of the fluid distribution media 108, 110 are exposed to theMEA 100.

The fluid distribution media 107 is preferably highly porous (i.e. about60%-80%), having a plurality of pores 120 formed within a body 121 ofthe fluid distribution media 108, 110. The plurality of pores 120comprise a plurality of internal pores 122 and external pores 124 thatare open to one another and form continuous flow paths or channels 126throughout the body 121 that extend from the first external surface 117to the second external surface 118 of the fluid distribution media 107.Internal pores 122 are located within the bulk of the fluid distributionmedia and external pores 124 end at the diffusion element surface. Asused herein, the terms “pore” and “pores” refers to pores of varioussizes, including so-called “macropores” (pores greater than 50 nmdiameter), “mesopores” (pores having diameter between 2 nm and 50 nm),and “micropores” (pores less than 2 nm diameter), unless otherwiseindicated, and “pore size” refers to an average or median valueincluding both the internal and external pore diameter sizes. It ispreferred that the average pore size be equivalent to a radius ofgreater than about 2 μm and less than about 30 μm. Since these openingsare disposed internally within the body 121 of fluid distribution medialayers (e.g. 108, 110) the surfaces of the openings are referred to asinternal surfaces 128, or the media interior.

According to the present invention, preferred non-metallic conductivefluid distribution media 107 comprises carbon. Such fluid distributionmedia is well known in the art, and preferably comprises carbon fiber orgraphite. The porous fluid distribution media 107 may be manufactured aspaper, woven cloth, non-woven cloth, fiber, or foam. One such knownporous fluid distribution media 107 comprises a graphite paper having aporosity of about 70% by volume, an uncompressed thickness of about 0.17mm, which is commercially available from the Toray Company under thetrade name Toray TGPH-060. Reactant fluids are delivered to the MEA 100via the fluid flow channels 126 within the first and second porous medialayers 108, 110, where the electrochemical reactions occur and generateelectrical current.

Electrical contact through an electrically conductive path at thecontact regions 116 is dependent upon the relative electrical contactresistance at an interface of the surfaces of the contacting elements.Although non-metallic fluid distribution media 107 is preferred for itscorrosion resistance, strength, physical durability in a fuel cellenvironment, and low bulk electrical resistance, it has been found thatthe interface between a metal substrate 113,115 and non-metal fluiddistribution media 107 can contribute to an increased electrical contactresistance at the interface due to the dissimilarity of the respectivematerials. It is believed that the molecular interaction between themetal and non-metal material at such an interface may increase thecontact resistance due to differences in the respective surface energiesand other molecular and physical interactions. Thus, one aspect of thepresent invention provides a conductive material coated on the materialcomprising the outer surfaces of the pores 120 of the porousnon-metallic fluid distribution media along surface 107 to formmetallized regions 130. The metallized regions 130 are formed along thefirst external surfaces 117 that confront the metal substrates 113,115.The metallized regions 130 integrated with the fluid distribution medialayer 107 at the first external surface 117 and have been demonstratedto sustainedly reduce contact resistance when compared with fluiddistribution media layers having no metal coating or metallized regions.It is preferred that the contact resistance of the electricallyconductive element of the present invention is less than 30 mOhm-cm² andmore preferably less than 15 mOhm-cm². Although not limiting to themanner in which the present operation operates, it is believed that theconductive metallized regions 130 at the contact surface 117 of thefluid distribution media 107 provide an improved electrical interface atthe contact regions 116 by contacting similar materials (i.e. metals)with correspondingly similar molecular and physical characteristics(e.g. surface energies). Further, it is believed that the metallizedregions 130 on the porous fluid distribution media 107 provide more evenelectrical current distribution through the body 121 of the media 107 asthe current approaches the discrete and non-continuous contact regions116 associated with the lands 131 of the flow field configuration on theseparator plate substrates 113,115.

In one preferred embodiment according to the present invention, themetallized regions 130 are applied along the external surface 117 of thefluid distribution media 107. The thickness of the metallized regions130 is less than 80 nm, preferably less than 50 nm, and more preferablybetween about 2 to about 10 nm. In certain preferred embodimentsaccording to the present invention, the thickness of the metallizedregions 130 is less than or equal to the depth of two atomic monolayersof the metal selected for the coating 130. A most preferred thickness ofthe metallized regions 130 is a monatomic layer, or an average thicknessequal to the diameter of about one atom of the coating material, or adepth of about one layer of atoms of the coating material. Such asthickness corresponds to a thickness of less than about 1 nm. Inparticular embodiments the thickness is from about 0.3 nm to about 0.5nm.

“Ultra-thin” layers of conductive metal deposited within the metallizedregions generally refers to thicknesses less than about 40 nm, morepreferably less than 15 nm, and most preferably less than or equal tothe depth of two atomic monolayers. In one particular embodiment of theinvention, the “ultra-thin” layers of conductive metal deposited withinthe metallized regions refers to layer having an average thickness equalto the diameter of about one atom of conductive metal.

It is preferred that the conductive metallized regions 130 also coat theexternal pore 124 surfaces and the surfaces 128 of the internal pores122 and extends into the body 121 of the fluid distribution media 107 ata depth of at least about 2 to about 10 nm. It is preferred that themetallized regions 130 are electrically conductive, oxidation resistant,and acid-resistant and in certain preferred embodiments the electricallyconductive material forming the metallized region comprises a noblemetal selected from the group consisting of: ruthenium (Ru), rhodium(Rh), palladium (Pd), silver (Ag), iridium (Ir), platinum (Pt), osmium(Os), and compounds and/or alloys thereof, Other preferred materials forthe metallized regions 130 include those that comprise chromium (Cr) orcompounds of Cr, such as chromium nitride (CrN). A most preferred metalfor the metallized regions 130 comprises gold (Au). As recognized by oneof skill in the art, the conductive metal composition may comprisemixtures of the above identified metals and/or metal nitrides. It shouldfurther be recognized that the metallized regions 130 can compriseconductive metal oxides, for instance, as non-limiting examples,ruthenium oxide (RuO₂), iridium oxide (IrO₂), fluorine doped tin oxide(SnO₂:F) and mixtures thereof.

In one alternate preferred embodiment of the present invention, shown inFIG. 5, discrete metallized regions 130 a of the porous media 107correspond to electrically conductive regions of the external surface117, and the non-metallized regions 133 correspond to the electricallynon-conductive regions. Electrically conductive regions include thoseareas that contact lands 131 and establish the electrically conductivepath at the contact regions 116. In other preferred embodiments, such asthat shown in FIG. 4, the metallized regions 130 cover the entiresurface of the external surface 117 which promotes more even currentdistribution into the body 121 of the porous media 107. In theembodiment with discrete metallized regions 130 a corresponding toelectrically active contact regions 116, the electrically non-conductiveand non-metallized regions of external surfaces 117 are covered ormasked while the conductive metal is applied. A mask is any materialthat is applied to a substrate and remains stable during coatingapplication. Often, mask materials are selected to permit recovery andrecycling of the metals deposited over the mask during the depositionprocess, and are well known in the art. Preferred mask materialscompatible with the present invention include, by way of example,metals, such as stainless steel and titanium, or silicon and aluminabased ceramics.

A variety of depositing methods may be employed to apply the conductivemetal compositions that form the metallized regions 130 of the fluiddistribution media 107. One preferred method of depositing theconductive metal of the metallized regions 130 onto the fluiddistribution porous media 107 will now be described with reference toFIG. 6. In order to deposit the conductive metal onto the substrate, anion-assisted, physical vapor deposition (PVD) method is employed.

In FIG. 6, an ion-assisted PVD apparatus 136 that is used to apply theconductive metal composition of the metallized regions 130 is shown. Theapparatus 136 includes a deposition chamber 138 and two electron guns, Aand B, for deposition of the metal coating. The apparatus 136 alsoincludes a turbo pump which allows the apparatus to operated in anultra-high vacuum. The substrate to be coated with the conductive metalis first placed in a “load-lock” chamber 137 where the pressure isbetween about 10⁻⁵ to 10⁻⁶ Torr or 1.3×10⁻³ Pa to 1.3×10⁻⁴ Pa. Thesubstrate is then transferred to the deposition chamber 138. Once thesubstrate is placed into the chamber 138, the pressure is lowered toabout 10⁻⁹ Torr (1.3×10⁻⁷ Pa). A first crucible 140 in the chamber holdsthe metal to be deposited. If a combination of metals or noble metals isto be deposited, a second metal is held by a second crucible 142. Forexample, the first crucible 140 contains a first metal (e.g. titanium)that is deposited as a first layer and crucible 142 contains a secondmetal (e.g. gold) which is deposited over the first layer, forming asecond layer. Another option available may be to deposit a combinationof metals simultaneously. Noble metals are deposited on the substrate ata rate of 0.10 nm/s to a thickness of less than 80 nm, which is observedby thickness monitors known in the art. The metallized regions 130 mayhave conductive metal deposited onto the substrate at ultra-lowthicknesses of less than 80 nm, preferably less 40 nm, more preferablyabout 2 to about 10 nm, and most preferably about 0.3 nm to about 0.5nm. When the metallized region 130 has a thickness of at least about 2nm, it is preferably that the loading is 0.02 mg/cm². It is possiblewith the present process to coat only a very thin layer (i.e. anultra-thin layer on the order of 10-20 nm), and preferably a monoatomiclayer on the order of about 0.3 to about 0.5 nm, thereby achieving goodsurface coverage, relatively uniform coverage, and good adhesion. Suchultra-thin layers are cost-effective and have now been found to beeffective even when monoatomic. Thus, the use of ion-assisted, PVDallows the electrically conductive material to be deposited on thesubstrate very smoothly, evenly, and in a lower-cost ultra-thin layer.

Another preferred method of applying a metal coating 130 according tothe present invention include electron beam evaporation, where thesubstrate is contained in a vacuum chamber (from between about 10⁻³ to10⁻⁴ Torr or about 1.3×10⁻¹ Pa to 1.3×10⁻² Pa) and a metal evaporant isheated by a charged electron beam, where it evaporates and thencondenses on the target substrate. One particular way to deposit amonoatomic layer is to impose an ultrahigh vacuum (UHV) on the chamberso as to prevent interactions with other atoms. Such interatomicinteractions cause a non-uniform deposition of the monoatomic layer. Asan illustrative example, an ultrahigh vacuum may include a pressure ofless than about 1×10⁻⁸ Torr (less than 1×10⁻⁶ Pa).

In a further embodiment of the invention, a preferred method of applyingmetallized regions 130 of the fluid distribution media 107 includesatomic layer deposition (ALD), also known as atomic layer epitaxy (ALE).ALD is a self-limiting method for chemically depositing or growingultra-thin films on a substrate. The method involves subjecting thesubstrate to self-saturating surface reactions. The surface reactionsmay be conducted sequentially and/or in an alternating fashion,depending on the composition and structure of the film desired. The ALDprocess is described in U.S. Pat. No. 4,058,430 of Suntola et al.,incorporated herein by reference.

An ALD apparatus may be characterized by a vacuum deposition chamberhaving a holder for a substrate, at least one vapor source (known as theprecursor) and controlled means by which the substrate may beindividually subjected to a vapor source. The controlled means mayinclude heaters, coolers and high speed valves for controlling theexposure of the substrate to the vapor source.

The ALD process for deposition of metallized regions 130 involvesreaction of the surface of the fluid distribution media 107 in adeposition chamber with a single vapor of an electrically conductivematerial or reaction of the surface with multiple vapors introduced oneat a time and consisting of the elementary components of theelectrically conductive material. The vapor may be pulsed into thevacuum deposition chamber on a carrier gas and may be quickly purged,for example, by vacuum pumping or flushing with an inert gas. Suchpulsing of the vapor and purging of the system may be performed tocontrol the dose of the precursor vapor to which the substrate isexposed.

Generally, the ALD process is performed at elevated temperatures andreduced pressures. It is important that the temperature of thedeposition chamber be high enough that reaction between the substrateand the precursor vapor occurs, while also preventing condensation ofthe vapor onto the surface. As nonlimiting examples, the reaction spacein the deposition chamber may be heated to between about 150° C. andabout 600° C., and the operating pressure may be between about 7.5×10⁻²Torr and about 4 Torr (about 1 Pa to about 5000 Pa).

As a result of ALD surface reactions, not more than one atomic layer ofthe electrically conductive material is bound to the surface, therebyproviding a monoatomic coating of electrically conductive material. Withsequential or alternating reactions, composite layers may be formed.Furthermore, additional atomic monolayers may be grown, therebyproviding a coating with a higher thickness. It should be understoodthat the electrically conductive material deposited by ALD may includemetal alloys and laminates, e.g. additional monoatomic layers.

As an illustrative example, a monoatomic layer coating comprising Au maybe deposited by ALD. Such a monoatomic layer may be deposited onto adistribution media surface by first pulsing a gold precursor vapor,selected from the family of organic and inorganic gold precursors orcombinations thereof. As a nonlimiting example, the gold precursor mayinclude dimethyl acetylacetonate Au. The precursor may be pulsed into adeposition chamber on an inert carrier gas such as, for example, argonor nitrogen. The chamber may subsequently be purged with a reactive gas,for example oxygen, resulting in a monoatomic layer coating of Au on thesurface of the distribution media.

Monoatomic layer coatings can be also be deposited by electrochemicalreactions. One particular example of an electrochemical form ofdeposition includes under-potential deposition (UPD). In general, UPD isa phenomenon where an element is deposited at a potential prior to (orunder) that needed to deposit the element onto itself. This effect canresult from, in certain instances, increased stability caused byreaction of the element with a first element present at the substratesurface. As a nonlimiting example of UPD, a second element may bereacted at a controlled potential with a previously deposited atomiclayer of a first element to form a single atomic layer of the desiredchemical species. The use of UPD provides increased control over thedeposited structure, morphology, and composition, and thus is useful indeposition of single atomic layers.

A further electrochemical method by which monoatomic layers may bedeposited, and which may also be used in conjunction with UPD, is knownin the art as displacement deposition. Electrochemical displacementdeposition reactions involve electrochemical precipitation of a secondmetal, from a solution including the salt of the second metal, onto asubstrate having a first and more reactive or electropositive metal. Thefirst metal, in turn, progressively dissolves and is displaced with thesecond metal. Thus, a monoatomic coating layer of one element depositedby any number of the aforementioned methods may be subsequentlydisplaced with another element by displacement deposition.

As an illustrative example of a displacement deposition process, a morereactive metal such as copper may be deposited as a monatomic layer byALD. The monolayer of copper may then be replaced by displacementdeposition with a noble metal, for example, gold. The displacementdeposition provides for contact of the copper surface with a dissolvedsalt of gold, for example, gold chloride. This contact allows the morereactive copper metal on the coated surface to dissolve in the solutionand be displaced with a monoatomic layer of gold.

An additional PVD method that may be suitable for the present inventionis magnetron sputtering, where a metal target (the conductive metal forthe metallized regions 130) is bombarded with a sputter gun in an argonion atmosphere, while the substrate is charged. The sputter gun forms aplasma of metal particles and argon ions that transfer by momentum tocoat the substrate. However, the use of ion-assisted PVD as previouslydescribed may provide better control of plasma than in magnetronsputtering because in sputtering the direction of the plasma may beharder to regulate. Ion-assisted PVD provides better control of thedeposition parameters because the ion beams have low energy and are wellcollimated, with divergence angles of only a few degrees. Due to thehigher difficulty in regulation when compared to PVD or ALD methods,however, magnetron sputtering may not be suitable for smooth and evendeposition of monoatomic coatings. It is recognized that various factorsmay promote the use of one application method over another, includingoverall processing time and cost.

The conductive metal of the metallized regions 130 may also be appliedby electroplating (e.g. electrolytic deposition), electroless plating,or pulse laser deposition. A higher difficulty in regulation and controlwith these methods, however, means that they may also not be suitablefor uniform deposition of monoatomic coatings, especially in comparisonto the aforementioned methods of physical vapor deposition, electronbeam evaporation, atomic layer deposition, underpotential deposition,and displacement deposition.

Preferred embodiments of the present invention provide a low contactresistance across the separator plate substrates 113,115 through theporous media 107 having the metallized regions 130. Further,electrically conductive elements according to the present invention donot require the removal of a passivation layer (i.e. metal oxide layer)from the metallic separator plate substrates 113,115 along contactsurfaces 132 prior to their incorporation into the conductive element ofthe present invention. Generally, a metal substrate 113,115 having anoxide layer that contacts a non-metallic fluid distribution layer(without metallized regions 130) creates an impermissibly highelectrical contact resistance. Thus, prior art methods of removing theoxide layer include a variety of methods, such as cathodic electrolyticcleaning, mechanical abrasion, cleaning the substrate with alkalinecleaners, and etching with acidic solvents or pickle liquors. Thepresent invention eliminates the necessity of removing the metal oxidesfrom the contact surfaces 132 of the metallic separator plate 113,115.

Thus, one preferred aspect of the present invention includes employingthe separator element substrate 113,115 comprising stainless steel,where the substrate surface 113,115 does not require the extensiveremoval of a passivation layer from the contact surface 132. Theimproved electrical conductivity at the interface at the contact regions116 provided by the metallized region coating 130 on the porous media107 permits use of metals in the separator element substrates 113,115that have a naturally occurring oxide layer at the contact surface 132.Hence, the present invention eliminates the costly and time intensivepre-processing step of removing metal oxides from the contact surface132 of the metal substrates 113,115. Further, higher grades of stainlesssteel previously discussed have a high corrosion resistance, and thuscan be used without any further protective treatment due to theirability to withstand the corrosive environment within the fuel cell.

The present invention is also suitable for use with separator plateelement substrates 113,115 that are coated with electrically conductiveprotective coatings that provide corrosion resistance to the underlyingmetal substrate 113,115. Such coatings may comprise oxidation andcorrosion resistant noble metal coating 130 layers (e.g. Au, Ag, Pt, Pd,Ru, Rh, Ir, Os, and mixtures thereof) or corrosion resistantelectrically conductive polymeric matrices, which generally compriseoxidation resistant polymers dispersed in a matrix of electricallyconductive corrosion resistant particles, as are known in the art. Theprotective coatings preferably have a resistivity less than about 50μohm-cm (Ω-cm) and comprise a plurality of oxidation-resistant,acid-insoluble, conductive particles (i.e. less than about 50 microns)dispersed throughout an acid-resistant, oxidation-resistant polymermatrix, where the polymer binds the particles together and holds them onthe surface 132 of the metal substrate 113,115. The coating containssufficient conductive filler particles to produce a resistivity nogreater than about 50 μohm-cm, and has a thickness between about 5microns and about 75 microns depending on the composition, resistivityand integrity of the coating. Cross-linked polymers are preferred forproducing impermeable coatings which protect the underlying metalsubstrate surface from permeation of corrosive agents.

Preferably, the conductive filler particles are selected from the groupconsisting of gold, platinum, graphite, carbon, nickel, conductive metalborides, nitrides and carbides (e.g. titanium nitride, titanium carbide,titanium diboride), titanium alloyed with chromium and/or nickel,palladium, niobium, rhodium, rare earth metals, and other nobel metals.Most preferably, the particles will comprise carbon or graphite (i.e.hexagonally crystallized carbon). The particles comprise varying weightpercentages of the coating depending on the density and conductivity ofthe particles (i.e., particles having a high conductivity and lowdensity can be used in lower weight percentages). Carbon/graphitecontaining coatings will typically contain 25 percent by weightcarbon/graphite particles. The polymer matrix comprises anywater-insoluble polymer that can be formed into a thin adherent film andthat can withstand the hostile oxidative and acidic environment of thefuel cell. Hence, such polymers, as epoxies, polyamide-imides,polyether-imides, polyphenols, fluro-elastomers (e.g., polyvinylideneflouride), polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics,and urethanes, inter alia are seen to be useful with the presentinvention. In such an embodiment, where the surfaces 132 are overlaidwith a protective coating, the metal substrates 113,115 comprise acorrosion-susceptible metal such as aluminum, titanium, or lower gradestainless steel that is coated with a corrosion resistant protectivecoating.

In certain embodiments of the present invention, it is preferred thatthe contact surface 132 of the separator element metal substrates113,115 has essentially clean surface, where loosely adheredcontaminants are removed, prior to incorporation into the electricallyconductive element. Such cleaning typically serves to remove any looselyadhered contaminants, such as oils, grease, waxy solids, particles(including metallic particles, carbon particles, dust, and dirt),silica, scale, and mixtures thereof. Many contaminants are added duringthe manufacturing of the metal material, and may also accumulate on thecontact surface 132 during transport or storage. Thus, cleaning of thecontact surface 132 of the metal substrate 113,115 is especiallypreferred in circumstances where the metal substrate 113,115 is soiledwith contaminants. Cleaning of the metal substrate 113,115 may entailmechanical abrasion; cleaning with traditional alkaline cleaners,surfactants, mild acid washes; or ultrasonic cleaning. The choice of theappropriate cleaning process or sequence of cleaning processes isselected based upon both the nature of the contaminant and the metal.

Experimental details regarding an illustrative embodiment of the presentinvention will now be described in detail. In this illustrativeembodiment, gold is chosen as the noble electrically conductive materialto be deposited by ion-assisted PVD onto Toray fluid distribution mediagraphite paper having a porosity of about 70% by volume, an uncompressedthickness of about 0.17 mm, which is commercially available from theToray Company, as the product Toray TGPH-060. In the first experiment,gold was deposited by PVD onto the Toray paper by a Teer magnetronsputter system. The magnetron targets were 99.99% pure Au. The Audeposition was done at 50V bias using 0.2 A for one minute to achieve agold coating 130 thickness of 10 nm.

As shown in FIG. 7, the Sample was prepared in the experiment describedabove and the Control is a non-coated prior art Toray 060 graphite paperhaving the same specifications as the Sample prior to the coatingprocess. The contact resistance was measured across both the Sample andControl through a 316L stainless steel flat plate through a range ofpressures. A surface area of 49 cm² was tested using 50 A/cm² currentwhich is applied by a direct current supply. The resistance was measuredusing a four-point method and calculated from measured voltage drops andfrom known applied currents and sample dimensions. The voltage drop wasmeasured “paper-to-paper” for both the Sample and Control, meaning anassembly was formed by sandwiching the steel plate between two diffusionmedia layers, where the voltage was measured across the assembly.Contact resistance measurements were measured as milli-Ohm per squarecentimeter (mΩ/cm²) with incremental force applied. The 316L stainlesssteel plates were not treated (i.e. no removal of oxide layers orcleaning), but rather used in the condition as received from themanufacturer. The paper without the gold coating 130 exhibits highcontact resistance values, with the lowest contact resistance value atapproximately 125 mOhm-cm² when the pressure applied is 400 p.s.i. (2700kPa). The Sample prepared in accordance with the present inventiondemonstrates significantly lower contact resistance (i.e. less thanapproximately 125 mOhm-cm²) through the interface at the contact regionsover across the entire contact surface and over the range of compressionpressures tested.

In FIG. 8, another comparison was performed between the same Sample andControl as in FIG. 7, however, the 316L stainless steel used in thecontact resistance measurement was machined with grooves along thecontact surface to form flow channels and lands (in a 1:1 ratio of landsto grooves), with a compression pressure measured for the entire surfacearea. Thus the electrical contact regions were thus formed at thediscrete land regions. The 316L stainless steel was otherwise untreated.As demonstrated across the range of applied pressures, the Sampleprepared according to the present invention was significantly lower incontact resistance than the prior art Control, and showed an evengreater improvement discrepancy between the sample and control contactresistance values (i.e. greater than 150 mOhm-cm² at the highestpressure tested of 300 p.s.i. or 2000 kPa) than that shown in FIG. 7above. Thus, conductive elements prepared in accordance with the presentinvention have an improved electrical interface between the non-metallicporous fluid distribution media and the metallic substrate of theseparator element. The metallized regions of the present inventionprovide an ultra-thin conductive metal coating that sufficiently coversthe surface of the porous fluid distribution element to provide a lowcontact resistance for an electrically conductive fluid distributionelement, which improves the overall performance of a fuel cell.Furthermore, the thickness of the metal coating is such that themanufacturing cost of preparing an electrically conductive fluiddistribution element is minimized. Processing costs are further reducedby eliminating the step of removing metal oxides from metal substratesthat will form an electrical interface with the fluid distributionelement. The improved electrical interface reduces contact resistanceand promotes more widespread and even current distribution, which willincrease the operational efficiency and overall lifetime of the membraneand the fuel cell stack.

The Sample described above may also be produced by alternate methodsincluding, for example, ion-assisted PVD and atomic layer deposition. ASample produced by such methods may have a monoatomic layer coatingcomprising gold. For example, in the ion-assisted PVD embodiment, goldis chosen as the noble electrically conductive metal to be deposited byion-assisted PVD onto Toray fluid distribution media graphite paperhaving a porosity of about 70% by volume, an uncompressed thickness ofabout 0.17 mm. The graphite paper is commercially available from theToray Company, as the product Toray TGPH-060. In this experiment, goldis deposited by PVD onto the Toray paper. The magnetron targets are99.99% pure Au. A single electron beam evaporation is used to depositthe gold coating 130 to a monoatomic thickness of approximately 03 nm to0.5 nm, at a rate of approximately 0.04 nm/s. The temperature duringdeposition does not exceed a temperature between about 35° C. and 40° C.and the deposition is completed in a time of less than 10 seconds.

In an illustrative atomic layer deposition (ALD) example, the goldcoating 130 on the Sample described above is applied onto Toray fluiddistribution media graphite paper by an ALD system. Samples of the Torayfluid distribution media graphite paper are placed into an ALDdeposition chamber. The reaction space in the deposition chamber isheated to a temperature greater than 150° C. at an operating pressurebetween about 7.5×10⁻² Torr and about 4 Torr. A gold precursorcomprising dimethyl acetylacetonate Au is pulsed into the depositionchamber on an argon carrier gas at a flow rate of about 50 ml/min toabout 200 ml/min, followed by a purge of oxygen gas to complete the ALDreaction. A single monoatomic layer coating on graphite paper, having athickness of approximately 0.3 nm to 0.5 nm, is thereby provided.

It has been further demonstrated that ultra-thin coatings at thediffusion media/bipolar plate interface exhibit an equally lowresistance in comparison to thicker coatings and have a negligibleinterface resistance in comparison to other parts of the fuel cellassembly. See, for example, U.S. Published Application No. 2005/0100771,herein incorporated by reference in its entirety. Thus, one of skill inthe art should appreciate that a Sample having a monoatomic gold layerexhibits a contact resistance substantially similar to that of theSample depicted in FIGS. 7 and 8.

The description of the above embodiments and method is merely exemplaryin nature and, thus, variations that do not depart from the gist of theinvention are intended to be within the scope of the invention. Suchvariations are not to be regarded as a departure from the spirit andscope of the invention.

1. An electrically conductive element for use in a fuel cell comprising:a conductive metal substrate having a major surface with surface oxidesformed thereon; a layer of conductive non-metallic porous media having asurface facing said major surface of said metal substrate, wherein saidlayer defines pores forming flow paths therethrough, said poresincluding internal pores and external pores; and one or more metallizedregions on said surface of said layer, each said metallized regioncontaining an electrically conductive material and having an averagethickness less than about 40 nm, wherein said electrically conductivematerial is deposited on interior surfaces of at least a portion of atleast one of said internal pores and said external pores in said one ormore metallized regions and extends into said layer, and wherein saidmajor surface of said conductive metal substrate is arranged in contactwith said one or more metallized regions to provide an electricallyconductive path from said layer through said one or more metallizedregions and said surface oxides to said metal substrate.
 2. Theelectrically conductive element according to claim 1, wherein each ofsaid metallized regions provides an increased electrical conductivity ascompared to a non-metallized region.
 3. The electrically conductiveelement according to claim 1, wherein said one metallized regionessentially entirely covers said surface of said layer.
 4. Theelectrically conductive element according to claim 1, wherein saidconductive metal substrate has a surface facing said layer which ispatterned with a plurality of grooves and lands, and wherein said landsare in contact with respective said metallized regions.
 5. Theelectrically conductive element according to claim 4, whereinsubstantially an entire surface of each said land is in contact with arespective said metallized region.
 6. The electrically conductiveelement according to claim 1, wherein said layer of conductivenon-metallic porous media includes one or more non-metallized regions onsaid surface of said layer, wherein said conductive metal substrate isin contact with said metallized regions and said non-metallized regions.7. The electrically conductive element according to claim 1, whereinsaid metallic substrate is selected from the group consisting ofstainless steel, aluminum, and titanium.
 8. The electrically conductiveelement according to claim 1, wherein said electrically conductivematerial of said metallized regions comprises a noble metal.
 9. Theelectrically conductive element according to claim 1, wherein saidelectrically conductive material of said metallized regions is selectedfrom the group consisting of: Cr, CrN, Ru, Rh, Pd, Ag, Ir, Pt, Os, Au,and mixtures thereof.
 10. The electrically conductive element accordingto claim 9, wherein said electrically conductive material comprises Au.11. An assembly for use in a fuel cell comprising: an electricallyconductive metal substrate having a major surface with surface oxidesformed thereon; a layer of electrically conductive porous fluiddistribution media having a first surface and a second surface, saidfirst surface facing said major surface of said metal substrate, whereinsaid layer defines pores forming flow paths therethrough, said poresincluding internal pores and external pores; a membrane electrodeassembly, said second surface of said layer confronting the membraneelectrode assembly; and one or more metallized regions on said firstsurface and said second surface of said layer, each said metallizedregion containing an electrically conductive material and having anaverage thickness less than about 40 nm, wherein said electricallyconductive material is deposited on interior surfaces of at least aportion of at least one of said internal pores and said external poresin said metallized regions and extends into said layer, wherein saidmajor surface of said conductive metal substrate is arranged in contactwith said metallized regions to provide an electrically conductive pathfrom said layer through said metallized regions and said surface oxidesto said metal substrate, and wherein an electrical contact resistanceacross said metal substrate through said metallized regions to saidlayer is less than a comparative contact resistance across a similarmetal substrate and a similar layer of fluid distribution media absentsaid metallized regions.
 12. A method for manufacturing an electricallyconductive element for a fuel cell, comprising: depositing anelectrically conductive material on a surface of a layer of electricallyconductive porous media to form one or more metallized regions andhaving an average thickness less than about 40 nm, wherein said layerdefines pores forming flow paths therethrough, said pores includinginternal pores and external pores, wherein said electrically conductivematerial is deposited on interior surfaces of at least a portion of atleast one of said internal pores and said external pores in said one ormore metallized regions and extends into said layer; positioning saidsurface having said metallized regions adjacent to a major surface of ametallic electrically conductive substrate, said major surface havingsurface oxides formed thereon, said surface facing said major surface;and contacting said substrate with said surface having said one or moremetallized regions to form an electrically conductive path from saidlayer through said one or more metallized regions and said surfaceoxides to said metal substrate.
 13. The method according to claim 12,wherein said depositing is conducted by a process selected from thegroup consisting of: electron beam evaporation, magnetron sputtering,physical vapor deposition, electrolytic deposition, electrolessdeposition, atomic layer deposition, underpotential deposition,displacement deposition, and combinations thereof.
 14. The methodaccording to claim 12, wherein said contacting is accomplished by acompressive force imparted on said fuel cell in an assembled fuel cellstack.
 15. The method according to claim 12, wherein said depositing isconducted by electron beam evaporation, and said electron beamevaporation is conducted under an ultra-high vacuum.
 16. The methodaccording to claim 12, wherein said depositing is conducted by atomiclayer deposition, and said atomic layer deposition comprises subjectingsaid substrate to at least one precursor vapor of said electricallyconductive material at a temperature sufficiently high for said vapor toreact with said surface of said substrate but too high for said vaporsto condense on said surface, and said atomic layer deposition isconducted at a temperature between about 150° C. and about 600° C. andat a pressure between about 7.5×10⁻² Torr and about 4 Torr.
 17. Themethod according to claim 12 wherein said depositing comprisesdisplacement deposition in which a second metal displaces a first metalon said surface of said electrically conductive porous media, and saiddisplacement deposition comprises immersing said porous media having alayer of said first metal in a solution having a salt of second metal,and said first metal dissolves and said second metal is deposited ontosaid porous media and said first metal is more reactive than said secondmetal.
 18. The method according to claim 17 wherein said first metal isselected from the group consisting of copper, zinc, iron and alloysthereof.
 19. The method according to claim 17 wherein said second metalis selected from the group consisting of gold, platinum, iridium,rhodium and alloys thereof.
 20. The method according to claim 12 whereinsaid depositing comprises underpotential deposition.